The appearance of the surface of objects in the real world is traditionally perceived through images that differ depending on the direction of observation, the direction of the illumination and the spectral and spatial distribution of the light over the surface of the object given by the resolution of the used optical systems, including the human eye. The goal of computer graphics is the creation of a virtual world, in which the same appearance of the object is achieved as it is perceived in the real world. In virtual reality, the world is presented as 3D objects covered with a suitable representation of their surface appearance. One of the methods used to achieve the high fidelity of the appearance of the surface structure is the bidirectional texture function (BTF) method. This method was introduced by the author K. J. Dana and collective in the publication “Reflectance and Texture of Real-world Surfaces”, published in the year 1999 by ACM Transactions on Graphics, in volume 18, issue 1, pp 1-34. The appearance of the object's surface here is represented by a group of thousands of images taken for various combinations of illumination and acquisition directions. The acquisition of BTF data is typically a time-demanding process and lasts up to several hours or even days depending on the chosen directional, spatial and spectral resolution of the individual BTF images.
The problem of obtaining a sufficient number of images with various combinations of the directions of illumination of the sample and the directions of data acquisition from the sample in the shortest possible time is resolved in various ways elsewhere in the world. There is, for example, a stationary device that deals with the problem of shortening the time required to acquire a sufficient number of images of the sample by using multiple sources of illumination of the sample or detectors acquiring the images of the sample, as shown in the publication by G. Müller and collective “Rapid synchronous acquisition of geometry and BTF for cultural heritage artefacts,” published in 2005 in the volume of the 6th International Symposium on Virtual Reality, Archaeology and Cultural Heritage (VAST) on pages 13-20, or in the publication by C. Schwartz and collective “Design and Implementation of Practical Bidirectional Texture Function Measurement Devices Focusing on the Developments at the University of Bonn,” published in 2014 in Sensors magazine, volume 14, issue 5, pp 7753-7819. Such devices, however, are stationary and bulky in order to ensure the mechanical stability of their entire construction. Furthermore, for such devices there is only a finite number of obtainable combinations of directions of illumination of the sample and directions of data acquisition from the sample due to the fixed positions of the light sources and detectors. For devices with a fixed grid of light sources and image sensors, the minimum difference of mutual directions of illumination of the sample and/or directions of the data acquisition from the sample is given by the size of the individual sources of illumination or image sensors so that they can be placed next to each other on a sphere, and the angular resolution of the directions can only be increased by increasing the radius of the sphere on which the sources and sensors are placed. This significantly limits the achieved angular resolution of the combinations of illumination and data acquisition directions for the construction of small, i.e. portable instruments for BTF measurements, for which a low weight is important from a utility perspective. Another design possibility involves moving the sample, as is mentioned in the publication of C. Schwartz and collective “Dome II: A Parallelized BTF Acquisition System”, published in June 2013 for the Eurographics Workshop on Material Appearance Modelling: Issues and Acquisition. Such a method is totally unsuitable for the implementation of portable devices for taking measurements in the field, where the sample is integrated into its environment and its extraction would lead to a breach of the placement of the sample.
There are also smaller devices, as presented, for example, in the publication by the authors Y. J. Han and K. Perlin, “Measuring bidirectional texture reflectance with a kaleidoscope,” published in 2003 in the magazine ACM Transactions on Graphics, issue 22 (3), pp 741-748, or in the publication by J. Filip and collective “Rapid material appearance acquisition using consumer hardware,” published in 2014 in the magazine Sensors, issue 14(10), pp 19785-19805. Nevertheless, these solutions generally have rather limited spatial resolutions of the acquired images and a small number of combinations of illumination and acquisition directions. The overall measurements, without the possibility of using the multiple acquisition of images, takes a very long time or, in the case of optical multiplication without the movement of individual parts of the device, only a small limited number of measurement directions is achieved.
Recently, in 2015, an article by the authors J. Hošek, V. Havran and collective, “Realisation of Circular Motion for Portable BTF Measurement Instrument,” was published in the magazine The Romanian Review Precision Mechanics, Optics & Mechatronics, issue 48, on pages 252-255, which equipped a spherical surface with LED diodes with an independently movable arm with several cameras. This made it possible to make the size of the instrument smaller thanks to the movability of the arm with the cameras, though the size of the device is still limited by the size of the cameras. The combinations of the directions of illumination, which comes from the light sources affixed to the spherical surface, and the imaging directions in this device are still quite limited, especially with the smaller dimensions of the overall device, which makes it possible to place only a small number of cameras on the arm. This then decreases the achieved angular resolution of combinations of illumination directions of the sample and the directions of data acquisition from the sample.
A glossmeter is known from the document WO 96/33401 A1 to YISUM RES DEV CO (IL), by RAPAPORT ERICH (IL), NUSSINOVITCH AMOS (IL). However, this glossmeter does not allow to minimize the size of the device, namely of the arch supporting the photodetectors, and requires the sample extraction from its environment. To achieve a sufficient number of combinations of illumination and acquisition direction, it is necessary to put the sample on a rotating plate.
US patent application US 2016/171748 A1 by KOIHLBRENNER ADRIAN (CH) ET AL, discloses a method and an apparition for digitizing the appearance of real material. Again, this invention requires the sample extraction from its environment, as the sample must be mounted on a rotating support.
US patent application US 2009/079987 by BEN-EZRA MOSHE ET AL describes a photodiode-based BRDF measurement. It includes a dome-like object with LED units to illuminate the sample. It allows in principle measurement of a sample without the sample extraction from its environment, as in our invention. Due to the use of LEDs only the device does not allow to measure spatially varying surface reflectance of a planar sample, as it measures reflectance for only a single point, known as BRDF. Replacing the LEDs by cameras with light sources would result in a device in the publication by C. Schwartz and collective “Design and Implementation of Practical Bidirectional Texture Function Measurement Devices Focusing on the Developments at the University of Bonn,” published in 2014 in Sensors magazine, volume 14, issue 5, pp 7753-7819. As after the replacement of LEDs by cameras the number of cameras in the device and the control units for the cameras would be rather high, it would require rather a rigid construction, increasing the weight of such a device substantially. As a result the weight of the device would be high that will be restrictive for an easy manipulation of the device during the adjustment of the device against the stationary sample. More importantly, for the measurement of the stationary sample the number of combinations of illumination directions and acquisition of a stationary sample is then fixed and limited, unlike our invention, as it is simply the number of illumination directions times the number of acquisition directions. Last but not least, in addition to the weight also the price of such a device would be rather high, unlike our proposed invention.